The Effect of the Laser Incidence Angle in the Surface of L-PBF

coatings

Article The Eﬀect of the Laser Incidence Angle in the Surface of L-PBF Processed Parts

Sara Sendino 1,* , Marc Gardon 2, Fernando Lartategui 3 , Silvia Martinez 1 and Aitzol Lamikiz 1 1 Aerospace Advance Manufacturing Research Center-CFAA, P. Tecnológico de Bizkaia 202, 48170 Zamudio, Spain; [email protected] (S.M.); [email protected] (A.L.) 2 EMEA AM Applications Manager, Renishaw Ibérica, Carrer de la Recerca 7, 08850 Barcelona, Spain; [email protected] 3 UO Structures & Statics, ITP Aero, P. Tecnológico de Bizkaia 300, 48170 Zamudio, Spain; [email protected] * Correspondence: [email protected]

Received: 29 September 2020; Accepted: 22 October 2020; Published: 24 October 2020

Abstract: The manufacture of multiple parts on the same platform is a common procedure in the Laser Powder Bed Fusion (L-PBF) process. The main advantage is that the entire working volume of the machine is used and a greater number of parts are obtained, thus reducing inert gas volume, raw powder consumption, and manufacturing time. However, one of the main disadvantages of this method is the possible differences in quality and surface finish of the different parts manufactured on the same platform depending on their orientation and location, even if they are manufactured with the same process parameters and raw powder material. Throughout this study, these surface quality differences were studied, focusing on the variation of the surface roughness with the angle of incidence of the laser with respect to the platform. First, a characterization test was carried out to understand the behavior of the laser in the different areas of the platform. Then, the surface roughness, microstructure, and minimum thickness of vertical walls were analyzed in the different areas of the platform. These results were related to the angle of incidence of the laser. As it was observed, the laser is completely perpendicular only in the center of the platform, whilst at the border of the platform, due to the incidence angle, it melts an elliptical area, which affects the roughness and thickness of the manufactured part. The roughness increases from values of Sa = 5.489 µm in the central part of the platform to 27.473 µm at the outer borders while the thickness of the manufactured thin walls increases around 40 µm.

Keywords: roughness; incidence angle; additive manufacturing; L-PBF; INCONEL718

1. Introduction Laser technology has been widely used in different industrial processes for many years, due to the characteristics of the laser source: high energy density, high efficiency, large temperature gradients, high repeatability of the process, and formation of a narrow heat-affected zone (HAZ) [1]. Nowadays, it is possible to find various research works on laser technologies such as laser welding [2,3], cutting [4,5], remelting [6,7], additive technologies [8,9] . . . thus demonstrating the importance of the development of these technologies. Due to the great development that the different technologies based on metal additive manufacturing (AM) have had in recent years, it seems reasonable that one of the most researched and promising metal AM process is the Laser Powder Bed Fusion (L-PBF) technology. This technology, also known by commercial names as Selective Laser Melting (SLM) or Direct Metal Laser Sintering (DMLS), is being applied for the direct manufacturing of functional components in different sectors [10,11].

Coatings 2020, 10, 1024; doi:10.3390/coatings10111024 www.mdpi.com/journal/coatings Coatings 2020, 10, 1024 2 of 12

In the L-PBF process, the final part is manufactured directly using powder as raw material. This powder is homogeneously distributed by a recoater in constant thickness layers of 20 to 60 µm. Once the recoater has distributed a layer of powder, a laser beam melts the powder within an area, which has been previously specified in the CAD (Computer-Aided Design) file. Once the complete layer has been processed, the platform lowers the thickness of a layer and the process is repeated layer-by-layer until the full part is obtained [12]. This technology presents many advantages compared to conventional machining technologies such as the ability to manufacture geometrically complex parts [10,13,14]. In addition, L-PBF also presents advantages above other AM technologies as Spierings [15] showed. One of these advantages is the wide range of alloys with very different properties and characteristics that can be processed. Thus, different applications and studies can be found with high toughness Ti-TiB composites [16,17], high-density alloys such as Cu-10Sn bronze [18], high-resistant alloys such as Al85Nd8Ni5Co2 [19], or high modulus of elasticity and mechanical strength CNTs/AlSi10Mg [20]. Therefore, L-PBF process is of special interest in sectors such as health and aerospace due to the possibility of obtaining complex parts in a wide range of available materials [10]. However, L-PBF technology has several limitations, particularly related to the poor surface finish and the need for final post-processing of the manufactured parts [10]. The parts manufactured using L-PBF show roughness values Ra between 3.2 and 12.5 µm [13], whereas conventional technologies obtain roughness values between 1–2 µm [11]. Surface quality is a critical attribute in the aerospace and medical sectors since surface imperfections caused by roughness could impact the performance of the part. Therefore, surface roughness is a well-researched crack initiator [12], reducing the life-span of the end part due to lower tensile and fatigue strength [10]. In the medical domain, rough surfaces can accumulate more micro-organisms than a smooth surface, requiring a finishing operation in many applications in the health sector [21]. Furthermore, roughness has a significant effect on the tribological behavior of the surface in applications where the friction between surfaces is present. Nowadays, in order to minimize the roughness and to achieve functional parts with real applicability, finishing operations are carried out. However, the finishing operation of parts manufactured using L-PBF technology is frequently very complex due to the geometrical complexity of the parts [22]. In addition, the finishing process increases considerably the manufacturing time and cost of the part [12] since most of the finishing processes involve manual operations. Moreover, sometimes it is not possible to finish some areas such as internal ducts or hollow cavities [23]. Therefore, it is necessary to reduce the roughness as much as possible in the additive manufacturing process itself before finishing the final part. The resultant roughness can be originated by different factors. On the one hand, the process parameters during manufacturing play an important role in the resulting roughness [22,24–26]. In this sense, it is necessary to adjust parameters such as laser power, exposure time, the distance between points, and overlapping. Some previous studies state that the use of higher laser power reduces roughness, since it increases the wettability of each layer [12], while other studies show that excessive laser power can lead to spatters of excessively oxidized particles, which would also increase roughness [23]. In addition, excessive laser power will lead to higher porosity. Thus, it is necessary to find a balance between the laser power and scan speed to ensure acceptable roughness and other properties such as porosity [11,12,23]. In addition, another process parameter that influences the roughness is the thickness of the layer used [27]. In any case, even when the optimal parameters are being used, the surface roughness can be very different depending on the position of the part in the machine [28], due to the direction of the inert gas in the manufacturing chamber [29], or the angle of incidence [28] and the geometry being processed. One of the most affected areas by roughness is the downskin area. Downskin areas are those that have not had molten material in the lower layer and have been built over non-melted powder. In this case, Coatings 2020, 10, 1024 3 of 12

Coatingssome of2020 these, 10, xpowder FOR PEER particles REVIEW remain partially attached to the processed layer when the layer3 meltsof 12 and re-solidifies during manufacturing [30]. ThereThere may may be be other other effects effects such such as as partial partial melting melting of of powder powder particles particles due due to to heat heat transfer transfer when when severalseveral parts parts are are manufactured manufactured on on the the same same platfo platform.rm. When When different different parts parts are are manufactured manufactured very very closeclose to to each each other, other, the the accumulated accumulated heat heat of of one one part part may may affect affect the the adjacent, adjacent, causing causing its its surface surface temperaturetemperature to to rise, rise, resulting resulting in in partially partially melt melt powder powder particles particles that that will will attach attach to the to thesurface surface [21]. [ 21A]. similarA similar effect eff ectcan can be beobserved observed in inthe the case case of oflarge large parts, parts, where where a ahigh high number number of of layers layers need need to to be be processedprocessed and and the the generated generated heat heat into into the the manufactur manufacturinging chamber chamber is is also also higher. higher. It It should should also also be be notednoted that thisthis roughness roughness due due to theto particlesthe particle attacheds attached to the surface to the will surface depend will on thedepend characteristics on the characteristicsof the powder of used the [powder31,32]. used [31,32]. Finally,Finally, a a very relevant factorfactor isis the the angle angle of of incidence incidence of of the the laser laser with with respect respect to the to powderthe powder layer. layer.Since Since the laser the laser beam beam is radiated is radiated from from the top the part top ofpart the of machine the machine and depending and depending on the on location the location of the ofprocessed the processed area, the area, laser the beam laser is focusedbeam is onfocused the powder on the bed powder at a certain bed incidenceat a certain angle incidence depending angle on dependingthe area where on the the area part where is being the manufactured.part is being manufactured. TheThe influence influence of thisthis eeffectffect has has not not been been so so analyzed analyzed by theby the literature literature in comparison in comparison with otherwith other effects effectssuch assuch powder as powder distribution distribution or process or process parameters. parameters. The study The study carried carried out by outKleszczynski by Kleszczynski et al. [ 28et ] al.analyses [28] analyses the impact the impact of increasing of increasing the radial the radial distance distance with respect with respect to the to center the center of the of platform, the platform, due to duethe to laser the beam laser incidencebeam incidence angle. Sinceangle. the Since laser the beam laser is beam not completely is not completely perpendicular perpendicular to the platform to the platformas can be as seen can be in seen Figure in1 Figure (figure 1 adapted(figure adapted from [ 33from,34]), [33,34]), an increase an increase in the in roughness the roughness of the of parts the partsis obtained. is obtained.

FigureFigure 1. 1. EffectEﬀect of of the the incidence angleangle ofof thethe laser: laser: ( a(a)) Diagram Diagram of of the the Laser Laser Powder Powder Bed Bed Fusion Fusion (L-PBF) (L- PBF)process process showing showing the scannerthe scanner operation operation and theand possible the possible angle angle of incidence of incidence in the in di theﬀerent different areas areas of the ofplatform; the platform; (b) Detail (b) Detail of part of 1 part located 1 located in the in central the central area of area the of platform the platform and part and 2 part located 2 located at one at side one of sidethe of platform. the platform.

ThroughoutThroughout this this article, article, the the effect eﬀect of of the the angl anglee of of incidence incidence of of the the laser laser beam beam on on the the surface surface roughnessroughness is is detailed, detailed, since, since, after after an an exhaustive exhaustive bib bibliographicliographic study, study, it it seems seems to to be be aa relevant relevant but but not not sufficientlysuﬃciently studied studied factor factor to to minimize minimize the the resulting resulting surface surface roughness roughness obtained obtained in in the the L-PBF L-PBF process. process.

2.2. Materials Materials and and Methods Methods TwoTwo different different tests tests were were designed designed to to analyze analyze the the incidence incidence angle angle and and check check its its effect effect on on the the final final partsparts manufactured manufactured using using L-PBF L-PBF technology. technology. All All te testssts were were carried carried out out using using the the Renishaw Renishaw AM400 AM400 manufacturingmanufacturing system system (Renishaw, (Renishaw, Stone, Stone, UK), UK), which which has has a a400 400 W W CW-M CW-M and and a a3D 3D scanner scanner for for laser laser beambeam movements movements with with a amaximum maximum scanning scanning speed speed of of 7000 7000 mm/s. mm/s. This This gives gives aa reduced reduced laser laser beam beam diameterdiameter of of 70 70 μµmm in in the the focal focal point point or or working working plane. plane. The The software software used used for for programming programming the the parameters of the manufacturing process was QuantAM (version V4). The tests designed for this purpose are summarized below:

2.1. First Tests: Study of the Morphology of the Laser on the Platform

Coatings 2020, 10, 1024 4 of 12 parameters of the manufacturing process was QuantAM (version V4). The tests designed for this purpose are summarized below:

Coatings 2020, 10, x FOR PEER REVIEW 4 of 12 2.1. First Tests: Study of the Morphology of the Laser on the Platform ThisThis study study analyzed analyzed the e fftheect ofeffect laser of radiation laser radiation on the di ffonerent the areasdifferent of the areas manufacturing of the manufacturing platform. To carryplatform. out thisTo carry test, theout laserthis test, beam the was laser radiated beam directlywas radiated to the directly platform, to withoutthe platform, pre-depositing without pre- a powderdepositing layer. a powder A grid oflayer. 25 A25 grid uniformly of 25 × 25 distributed uniformly pointsdistributed was designedpoints was in designed order to testin order the to × entiretest workspace the entire workspace of the platform. of the The platform. parameters The para weremeters adjusted were to adjusted analyze to the analyze points the marked points on marked the platformon the separately platform separately without any without overlapping, any overlapping, as shown inas Figure shown2. in In Figure addition, 2. In the addition, correct balancethe correct betweenbalance the between power andthe exposurepower and time exposure parameters time wasparameters set to optimize was set theto optimize mark on the the mark platform. on the Specifically,platform. a Specifically, power of 200 a power W, an exposureof 200 W, time an exposure of 200 µs, time and of a distance200 μs, and between a distance points between of 900 µpointsm wereof selected.900 μm were selected.

FigureFigure 2. Test 2. designTest design to analyze to analyze the inﬂuence the influence of laser inclination of laser angleinclination on the platform:angle on (athe) Distribution platform: (a) of theDistribution marks in of the the platform.; marks in ( bthe) Marks platform.; made (b) on Marks the platform made on in the a platform star shape in deﬁninga star shape the laserdefining strategythe laser used. strategy used.

OnceOnce the pointsthe points were markedwere marked in the di infferent the areasdifferent of the areas platform, of the the platform, morphology the and morphology dimensions and of thesedimensions points wereof these studied points using were an infinitestudied focus using microscopy an infinite (Infinite focus Focusmicroscopy Microscope (Infinite model Focus Control Server FP G1 Vf2, Alicona, Raaba/Graz, Austria), using 500 magnification. Microscope model Control Server FP G1 Vf2, Alicona, ×Raaba/Graz, Austria), using 500× magnification.The different marks were made in a star shape and distributed evenly throughout the platform. With thisThe star different shape, it marks was intended were made to change in a star the shape direction and distributed of the laser evenly in order throughout to be able tothe analyze platform. theWith distortion this star caused shape, by it the was angle intended of incidence to change independently the direction of of the this laser direction in order of theto be laser able and to analyze the possiblethe distortion delays related caused to by the the scanner. angle of incidence independently of this direction of the laser and the possibleFrom each delays of the related stars to marked the scanner. on the platform, 10 of the points were analyzed and measured by the softwareFrom Alicona each of MeasureSuitthe stars marked of the on Infinite the platform, Focus Microscope10 of the points measurement were analyzed system. and Thanksmeasured to by thisthe system, software it was Alicona possible MeasureSuit to estimate of the two Infinite radiuses Focus from Microscope each of the measurement elliptical or circularsystem. points,Thanks to andthis by dividingsystem, it these was twopossible radiuses, to estimate the aspect two ratio radiuses of the from ellipse each could of the be elliptical calculated, or ascircular can be points, seen in and Equationby dividing (1). In these the case two of radiuses, circular marks,the aspect the valueratio of of the this ellipse ratio was could equal be tocalculated, one. as can be seen in Equation (1). In the case of circular marks, the valueEllipse of this major ratio axis was equal to one. Ellipse aspect ratio = (1) EllipseEllipse minor major axis axis Ellipse aspect ratio (1) Ellipse minor axis Thanks to this study, it was possible to analyze the circularity of the marks made by the laser on the differentThanks parts to ofthis the study, platform. it was possible to analyze the circularity of the marks made by the laser on the different parts of the platform.

2.2. Second Test: Effect of the Angle of Incidence of the Laser on Parts Manufactured by L-PBF Technology Inconel 718 powder specially designed for this technology was used to manufacture these test parts. The powder shows high sphericity (over 80% of the particles showed 100% sphericity) and a particle size between 15–45 μm. Because of these specifications, the powder can flow and distribute correctly in each layer. In addition, powder meets the specific chemical composition of Inconel 718, shown in Table 1.

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2.2. Second Test: Effect of the Angle of Incidence of the Laser on Parts Manufactured by L-PBF Technology Inconel 718 powder specially designed for this technology was used to manufacture these test parts. The powder shows high sphericity (over 80% of the particles showed 100% sphericity) and a particle size between 15–45 µm. Because of these specifications, the powder can flow and distribute correctlyCoatings 2020 in, 10 each, x FOR layer. PEER In REVIEW addition, powder meets the specific chemical composition of Inconel5 of 718, 12 shown in Table1. Table 1. Chemical composition in weight percentage of Inconel 718.

Table 1. Chemical composition in weight percentage of InconelSi and 718. P and Elements Ni Cr Co C Mo Al Ti Fe Nb Cu B Mn S Elements Ni Cr Co C Mo Al Ti Fe Nb Si and Mn P and S Cu B 50– 17– 2.8– 0.2– 0.65– 4.75– %Weight%Weight 50–55 17–22 1 ≤1 0.08≤0.08 2.8–3.3 0.2–0.8 0.65–1.15 Bal Bal4.75–5.5 0.35≤0.35 ≤0.015 ≤0.30.3 ≤0.006 55 22 ≤ ≤ 3.3 0.8 1.15 5.5 ≤ ≤ ≤ ≤

ThreeThree didifferentfferent geometriesgeometries werewere designeddesigned andand analyzedanalyzed in order to study three different different eeffects:ffects: SurfaceSurface roughness,roughness, internalinternal microstructure,microstructure,and andthin thin walls walls thickness. thickness.

2.2.1.2.2.1. TestTest PartPart A:A: RoughnessRoughness MeasurementMeasurement (Red(Red Parts)Parts) TheseThese partsparts werewere designeddesigned specificallyspecifically forfor measuringmeasuring surfacesurface roughness,roughness, usingusing anan infiniteinfinite focusfocus 3D measurement system with 500 magnification, according to ISO 25178 [35]. 3D measurement system with 500×× magnification, according to ISO 25178 [35]. The geometry designed for this purpose had a study surface of 13 mm 13 mm, where three The geometry designed for this purpose had a study surface of 13 mm ×× 13 mm, where three measurements of 1 mm 13 mm on each of these surfaces were made, always analyzing the outside measurements of 1 mm ×× 13 mm on each of these surfaces were made, always analyzing the outside surfacesurface ofof each part part as as can can be be seen seen in in Figure Figure 3.3 .The The arithmetical arithmetical mean mean height height of the of the surface surface (Sa) ( andSa) andthe maximum the maximum height height of the of surface the surface (Sz) values (Sz) values were wereobtained obtained and the and standard the standard deviation deviation of these of thesemeasurements measurements was wascalculated calculated to determine to determine the theerro errorr bars. bars. In Inaddition, addition, thanks thanks to to this opticaloptical measurementmeasurement system,system, surface surface topographies topographies could could be obtained be obtained and thus and the thus particles the particles partially adheredpartially toadhered the surface to the could surface be alsocould analyzed. be also analyzed.

FigureFigure 3.3. DesignDesign ofof thethe secondsecond platform:platform: ((aa)) partsparts forfor thethe roughnessroughness measurement;measurement; ((bb)) partsparts forfor thethe microstructuremicrostructure analysis;analysis; ((cc)) parts parts for for measuring measuring the the thickness thickness of of thin thin walls. walls.

TheThe processprocess parametersparameters werewere thethe usualusual setset forfor thethe manufacturemanufacture ofof InconelInconel 718718 partsparts usingusing L-PBFL-PBF technologytechnology using using a a layer layer thickness thickness of of 60 60µ m.μm. Speciﬁcally, Specifically, the the volume volume parameters parameters were were a laser a laser power power of 200of 200 W, exposureW, exposure time time of 70 ofµs, 70 a distanceμs, a distance point andpoint hatch and distance hatch distance of 80 µm, of a 80 strategy μm, a anglestrategy variation angle ofvariation 67◦, and of the 67°, border and parametersthe border parameters were a laser were power a oflaser 125 power W, exposure of 125 timeW, exposure of 75 µs, atime point ofdistance 75 μs, a point distance of 20 μm, and a layer thickness of 60 μm. These parts were distributed on a platform with a radial distance of 0, 40, 75, 105, and 150 mm from the center.

2.2.2. Test Part B: Microstructure Analysis (Blue Parts) These parts were specially designed to analyze their internal microstructure. The geometry used was the same as for Parts A but the parameters were changed in order to study the desired effect. Specifically, these parts were manufactured without borders and using a 0° variation of the strategy

Coatings 2020, 10, 1024 6 of 12 of 20 µm, and a layer thickness of 60 µm. These parts were distributed on a platform with a radial distance of 0, 40, 75, 105, and 150 mm from the center.

2.2.2. Test Part B: Microstructure Analysis (Blue Parts) These parts were specially designed to analyze their internal microstructure. The geometry used Coatings 2020, 10, x FOR PEER REVIEW 6 of 12 was the same as for Parts A but the parameters were changed in order to study the desired effect. Specifically, these parts were manufactured without borders and using a 0 variation of the strategy angle so that the Gaussian-shape melted areas were aligned in order to analyze◦ them. These parts angle so that the Gaussian-shape melted areas were aligned in order to analyze them. These parts were distributed on a platform with a radial distance of 20, 75, 105, and 145 mm from the center. were distributed on a platform with a radial distance of 20, 75, 105, and 145 mm from the center. To analyze these parts, a section was cut and encapsulated. To analyze always the same surface, To analyze these parts, a section was cut and encapsulated. To analyze always the same surface, all parts were cut in the same direction (as shown in Figure 3b) and these samples were encapsulated all parts were cut in the same direction (as shown in Figure3b) and these samples were encapsulated using phenolic resin. using phenolic resin. Then, the planar grinding step was made using Silicon carbide and corundum sandpaper of Then, the planar grinding step was made using Silicon carbide and corundum sandpaper of lower lower grain size of FEPA (Federation of European Producers of Abrasives) 400, to achieve a surface grain size of FEPA (Federation of European Producers of Abrasives) 400, to achieve a surface ready to ready to be polished, four steps were made (using FEPA 400, FEPA 600, FEPA 800, and FEPA 1200 be polished, four steps were made (using FEPA 400, FEPA 600, FEPA 800, and FEPA 1200 grain size). grain size). Once the surface was ready, it was polished using a diamond polycrystalline of 1 and 3 Once the surface was ready, it was polished using a diamond polycrystalline of 1 and 3 µm. μm. Once the mirror-finished surface was achieved, this surface was chemically attacked using the Once the mirror-finished surface was achieved, this surface was chemically attacked using the 2525 Marble Marble etchant etchant using using the the swabbing swabbing method, method, as as sp specifiedecified in in the the ASTM ASTM E E 407 407 standard standard [36] [36] and and with with a composition of 4 g CuSO4, 20 cc HCl, and 20 cc H2O. Finally, surface images were taken using the a composition of 4 g CuSO4, 20 cc HCl, and 20 cc H2O. Finally, surface images were taken using the infinite focus 3D measurement system with 200 magnification. infinite focus 3D measurement system with 200×× magnification. 2.2.3. Test Part C: Thin Walls Thickness Measurements (Green Parts) 2.2.3. Test Part C: Thin Walls Thickness Measurements (Green Parts) Finally, thin walls of 13 mm height were designed and placed in different areas of the platform Finally, thin walls of 13 mm height were designed and placed in different areas of the platform to measure their thickness. These test parts were built with a single laser track, and the same border to measure their thickness. These test parts were built with a single laser track, and the same border parameters used in the “A Test Parts”. parameters used in the “A Test Parts”. Once these parts were manufactured, the thickness of each test part was measured using the Once these parts were manufactured, the thickness of each test part was measured using the infinite focus 3D measurement system with 200 magnification. Figure4 shows the methodology infinite focus 3D measurement system with 200×× magnification. Figure 4 shows the methodology used for the analysis of thin walls. The Image J image analysis software (version ImageJ 1.x [37]) was used for the analysis of thin walls. The Image J image analysis software (version ImageJ 1.x [37]) was used for extracting the contour of the images of the infinite focus microscope. This contour was then used for extracting the contour of the images of the infinite focus microscope. This contour was then approximated to ellipses to define the thickness of the part, which would be equivalent to the average approximated to ellipses to define the thickness of the part, which would be equivalent to the average value of the vertical radius of the ellipses. value of the vertical radius of the ellipses.

FigureFigure 4. 4. TheThe methodology methodology used used for for the the analysis analysis of of thin thin walls: walls: (a (a) )image image obtained obtained through through the the infinite inﬁnite focusfocus microscope; microscope; ( (bb)) extraction extraction of of the the thin thin wall wall contour; contour; ( (cc)) approximation approximation of of the the ellipses ellipses to to the the generatedgenerated contour; contour; ( (dd)) checking checking this this approximatio approximationn in the obtained image.

Twenty-fiveTwenty-ﬁve measurements measurements were were made made on on each each of of the the thin thin walls walls and and the the average average value value and and standardstandard deviation deviation between between them them were were calculated; calculated; th therefore,erefore, and and taking taking into into account account that that eight eight thin thin wallswalls were were manufactured manufactured for for each each radial radial distance, distance, 200 200 measurements measurements were were obtained obtained in in each each of of the the radialradial distances distances (these (these radial radial distances distances being being 20, 20, 70, 70, 100, 100, and and 145 145 mm). mm).

3. Results

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3. Results

3.1. First Tests: Study of the Morphology of the Laser on the Platform Analyzing the laser marks on the platform surface, it could be seen that these marks in the central part of the platform present a circular shape while marks located far from the center acquire an elliptical appearance. In addition, all the ellipses showed its largest radius in the radial direction towards the center of the platform. Figure5a shows this e ffect on the images obtained when analyzing the center point of stars with Coatings 2020, 10, x FOR PEER REVIEW 7 of 12 different positioning on the platform. As can be seen, the central area of the platform shows circular marks, with a diameter of around 200 µm, while in the case of the marks located away from the centre, marks, with a diameter of around 200 μm, while in the case of the marks located away from the elliptical marks up to 240 µm are observed. Therefore, a considerable difference between the different centre, elliptical marks up to 240 μm are observed. Therefore, a considerable difference between the laser marks on the platform is observed depending on the location of the laser mark. different laser marks on the platform is observed depending on the location of the laser mark.

FigureFigure 5. 5. AnalysisAnalysis of of the the marks marks made made by by the the laser laser on on the the platform: platform: ( (aa)) diagram diagram of of some some of of the the images images obtainedobtained when when at at the the central central mark mark of ofthe the stars stars wa wass analyzed analyzed (2-B, (2-B, 2-H, 2-H, 2-M, 2-M, 8-M, 8-M, 8-H, 8-H, and and3-M); 3-M); (b) ellipse(b) ellipse aspect aspect ratio ratio of different of diﬀerent stars stars analyzed. analyzed.

InIn addition, addition, the the ellipse ellipse aspect aspect rati ratioo was was calculated. calculated. It It is is observed observed in in Figure Figure 55bb that this ratio was closercloser to to unity in thethe teststests nearnear thethe center center of of the the platform, platform, while while in in the the marks marks located located on on the the sides sides of theof theplatform, platform, this this ratio ratio increase, increase, and and the the circularity circularity of the of the marks marks decrease. decrease.

3.2.3.2. Second Second Test: Test: Effect Eﬀect of of the the Angle Angle of of Inci Incidencedence on on Parts Parts Manufactured Manufactured by by L-PBF L-PBF Technology 3.2.1. Test Part A: Roughness Measurement 3.2.1. Test Part A: Roughness Measurement The results obtained with the roughness measurement are shown in Figure6. The results obtained with the roughness measurement are shown in Figure 6. Despite Despite manufacturing the same part with the same parameters and the same raw powder, manufacturing the same part with the same parameters and the same raw powder, surface roughness surface roughness is inﬂuenced by the position of the part on the platform. As it can be observed, is influenced by the position of the part on the platform. As it can be observed, higher surface higher surface roughness is measured for parts located away from the center of the platform. Results roughness is measured for parts located away from the center of the platform. Results show that the show that the roughness is dependent on the position of the part on the platform because of the angle roughness is dependent on the position of the part on the platform because of the angle of incidence of incidence of the laser. of the laser.

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marks, with a diameter of around 200 μm, while in the case of the marks located away from the centre, elliptical marks up to 240 μm are observed. Therefore, a considerable difference between the different laser marks on the platform is observed depending on the location of the laser mark.

Figure 5. Analysis of the marks made by the laser on the platform: (a) diagram of some of the images obtained when at the central mark of the stars was analyzed (2-B, 2-H, 2-M, 8-M, 8-H, and 3-M); (b) ellipse aspect ratio of different stars analyzed.

In addition, the ellipse aspect ratio was calculated. It is observed in Figure 5b that this ratio was closer to unity in the tests near the center of the platform, while in the marks located on the sides of the platform, this ratio increase, and the circularity of the marks decrease.

3.2. Second Test: Effect of the Angle of Incidence on Parts Manufactured by L-PBF Technology

3.2.1. Test Part A: Roughness Measurement The results obtained with the roughness measurement are shown in Figure 6. Despite manufacturing the same part with the same parameters and the same raw powder, surface roughness is influenced by the position of the part on the platform. As it can be observed, higher surface roughness is measured for parts located away from the center of the platform. Results show that the roughness is dependent on the position of the part on the platform because of the angle of incidence Coatingsof the2020 laser., 10, 1024 8 of 12

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FigureFigure 6. RoughnessRoughness results results obtained: obtained: (a (a) )image image and and topographies topographies of of maximum maximum and and minimum minimum roughness values: maximum roughness (225 and radial distance of 150 mm) and minimum roughness roughness values: maximum roughness (225°◦ and radial distance of 150 mm) and minimum roughness(center 0◦); (center (b) mean 0°); value (b) mean of the value roughness of the forroughness diﬀerent for radial different distance radial from distance the center from of the the center platform; of the(c) roughnessplatform; ( meanc) roughness value results mean obtainedvalue results each obtained test part. each test part.

As it is observed in Figure6, the relatively high roughness and poor surface finish of the As it is observed in Figure 6, the relatively high roughness and poor surface finish of the parts parts located at the borders of the platform can be seen with the naked eye. Analyzing the surface located at the borders of the platform can be seen with the naked eye. Analyzing the surface topographies, it could see how in the parts manufactured in the central part of the platform the layers topographies, it could see how in the parts manufactured in the central part of the platform the layers of the part and some particles partially adhered to the surface can be differentiated; however, in the of the part and some particles partially adhered to the surface can be differentiated; however, in the parts located in the outer border of the platform, neither the layers nor the particles partially adhered parts located in the outer border of the platform, neither the layers nor the particles partially adhered to the surface can be differentiated so clearly, but the surface quality has considerably worsened due to to the surface can be differentiated so clearly, but the surface quality has considerably worsened due the melting of the powder. to the melting of the powder. It has also been determined that the roughness does not increase linearly, but increases significantly It has also been determined that the roughness does not increase linearly, but increases at the borders of the platform while the roughness remains at similar values in the center. significantly at the borders of the platform while the roughness remains at similar values in the center. 3.2.2. Test Part B: Microstructure Analysis 3.2.2. Test Part B: Microstructure Analysis The microstructure has been also studied to analyze the possible influence of the incident angle The microstructure has been also studied to analyze the possible influence of the incident angle on part integrity. Figure7 shows the microstructure of two sections corresponding to the central area on part integrity. Figure 7 shows the microstructure of two sections corresponding to the central area and the peripheral border of the platform. While the section corresponding to the part in the central and the peripheral border of the platform. While the section corresponding to the part in the central area shows a uniform pattern of laser tracks, with the melted areas oriented in a vertical direction, area shows a uniform pattern of laser tracks, with the melted areas oriented in a vertical direction, the part in the border shows a less uniform pattern with the melted areas distorted due to the effect of the part in the border shows a less uniform pattern with the melted areas distorted due to the effect the laser orientation. This effect is shown schematically in Figure7. of the laser orientation. This effect is shown schematically in Figure 7.

FigureFigure 7. 7. AnalysisAnalysis of ofthe themicrostructure microstructure in th ine different the diﬀerent parts parts located located at 45°: at (a 45) central◦:(a) centralpart (radial part distance(radial distance from the from center the 20 center mm); 20(b) mm); Part located (b) Part at located the edge at theof the edge platform of the platform(radial distance (radial from distance the centerfrom the 145 center mm). 145 mm).

Nevertheless, although the laser pattern creates a different microstructure due to the angle of the laser beam, it is observed that both components are free of porosity and defects and the mechanical properties of both specimens meet the required requirements.

3.2.3. Test Part C: Thin Walls Thickness Measurements To conclude this study, thin walls corresponding to Test Part C and manufactured using single laser tracks were analyzed. The results of these tests are shown in Figure 8. Figure 8a shows the effect of the angle of incidence on the thickness for the different test parts manufactured in one of the diagonals of the platform (at 45°, according to Figure 3).

Coatings 2020, 10, x FOR PEER REVIEW 8 of 12

Figure 6. Roughness results obtained: (a) image and topographies of maximum and minimum roughness values: maximum roughness (225° and radial distance of 150 mm) and minimum roughness (center 0°); (b) mean value of the roughness for different radial distance from the center of the platform; (c) roughness mean value results obtained each test part.

As it is observed in Figure 6, the relatively high roughness and poor surface finish of the parts located at the borders of the platform can be seen with the naked eye. Analyzing the surface topographies, it could see how in the parts manufactured in the central part of the platform the layers of the part and some particles partially adhered to the surface can be differentiated; however, in the parts located in the outer border of the platform, neither the layers nor the particles partially adhered to the surface can be differentiated so clearly, but the surface quality has considerably worsened due to the melting of the powder. It has also been determined that the roughness does not increase linearly, but increases significantly at the borders of the platform while the roughness remains at similar values in the center.

3.2.2. Test Part B: Microstructure Analysis The microstructure has been also studied to analyze the possible influence of the incident angle on part integrity. Figure 7 shows the microstructure of two sections corresponding to the central area and the peripheral border of the platform. While the section corresponding to the part in the central area shows a uniform pattern of laser tracks, with the melted areas oriented in a vertical direction, the part in the border shows a less uniform pattern with the melted areas distorted due to the effect of the laser orientation. This effect is shown schematically in Figure 7.

Figure 7. Analysis of the microstructure in the different parts located at 45°: (a) central part (radial Coatingsdistance2020, 10 from, 1024 the center 20 mm); (b) Part located at the edge of the platform (radial distance from the9 of 12 center 145 mm).

Nevertheless,Nevertheless, althoughalthough thethe laser laser pattern pattern creates creates a dia ﬀdifferenterent microstructure microstructure due due to theto the angle angle of the of laserthe laser beam, beam, it is observed it is observed that both that components both compon areents free ofare porosity free of and porosity defects and and defects the mechanical and the propertiesmechanical of properties both specimens of both meet specim theens required meet the requirements. required requirements.

3.2.3. Test Part C: Thin Walls Thickness Measurements 3.2.3. Test Part C: Thin Walls Thickness Measurements To conclude this study, thin walls corresponding to Test Part C and manufactured using single To conclude this study, thin walls corresponding to Test Part C and manufactured using single laser tracks were analyzed. The results of these tests are shown in Figure8. Figure8a shows the laser tracks were analyzed. The results of these tests are shown in Figure 8. Figure 8a shows the effect eﬀect of the angle of incidence on the thickness for the diﬀerent test parts manufactured in one of the of the angle of incidence on the thickness for the different test parts manufactured in one of the diagonals of the platform (at 45 , according to Figure3). diagonals of the platform (at 45°,◦ according to Figure 3).

Figure 8. Analysis of the thin walls manufactured at 45◦:(a) Measurements of wall thickness for test parts manufactured at 45◦ and a diﬀerent distance from the platform center; (b) Mean value of the thickness for diﬀerent radial distances from the center of the platform.

The inﬂuence of the radial distance from the center of the platform in the thickness is observed in Figure8b) where the results show a gradual increase of the thickness as the radial distance increases. In particular, the thickness of these walls increased more than 40 µm in the parts located at the borders of the platform. Moreover, this result is consistent with the analysis of the previous tests.

4. Discussion and Conclusions On the one hand, the measurement of the geometry of the laser spot at the platform surface demonstrated that this spot is not circular at the borders of the platform and takes an elliptical shape, which causes position-dependent laser radiation and a distortion in the melt pool. This effect is due to the technology itself, the lens used in the scanner, and the physics of the process. In this technology, and in general, in all laser technologies that require a flat working field, F-theta lenses are used, which ensure that all the main beams on the image side are parallel to the optical axis, resulting in the perpendicularity of the beam with respect to the platform. On the other hand, results show that this angle of incidence influence the surface quality, microstructure, and thickness of the manufactured parts. As a consequence of the elliptical shape of the laser spot on the areas further away from the center, the area of powder bed heated is different from the central area and the melting of this powder does not occur in the same way. This effect is observed in Test Part A. Furthermore, after the analysis of Test Part C corresponding to the thin walls, the effect of the angle of incidence could also be seen, as the distortion of the laser spot has resulted in a larger melted area, which has increased the thickness of the walls of the parts located in the outer borders of the platform. Finally, after this exhaustive study, it has been possible to see how this angle of incidence has a relevant effect on the roughness of the part (Test Part B). It should also be noted that this effect can have a considerable effect, as the increase in the diameter of the laser spot on the platform can reach up to 40 µm and different effects can occur in the manufactured part: Coatings 2020, 10, 1024 10 of 12

Increase of the thickness of the thin walls: These walls were manufactured with a single laser • track and the thickness increase was very similar to the laser spot size enlargement at the borders of the platform. Roughness increase: Similarly, the results of the roughness measurements are also coherent with • the results of the other tests. Thus, an increase in roughness is observed as the test parts are manufactured away from the center of the platform. The high roughness, in this case, is due to the poor surface quality caused by the non-circular shape of the melt pool. Regarding microstructure, although a diﬀerent pattern is observed due to the inclination angle • and the distortion of the melt pool, similar mechanical properties are obtained, and no porosity or cracks have been observed.

Due to the great eﬀect of the angle of incidence on the diﬀerent parts manufactured and on the surface quality of the parts, it has been decided to continue with this study in future investigations. For this purpose, it is expected to characterize the laser spot on the platform, in order to determine this angle of incidence with high precision. Next, based on the shape and characteristics of the spot, the melt pool generated will be analyzed to try to obtain a direct relationship between the angle of incidence and the roughness of the parts.

Author Contributions: Conceptualization, S.S., S.M., and A.L.; methodology,S.S., S.M., M.G. and F.L.; experimental test S.S. and S.M.; result analysis, S.S., S.M., M.G., F.L. and A.L.; resources, S.M. and A.L.; writing—original draft preparation, S.S. and S.M.; writing—review and editing, S.S., S.M. and A.L.; supervision, M.G., F.L. and A.L.; project administration, M.G., F.L. and A.L.; funding acquisition, A.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Spanish Ministry of Industry and Competitiveness under the CDTI JANO project and by Basque Government (Eusko Jaurlaritza) under the ELKARTEK Program, QUALYFAM project, Grant No. KK-2020/00042. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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